One surprising consequence of quantum mechanics is the nonlocal correlation of a multi-particle system measured by joint-detection of distant particle detectors. In two publications by R. Meyers, K. S. Deacon, Y. H. Shih, entitled “Ghost Imaging Experiment by Measuring Reflected Photons,” Phys. Rev. A, Rapid Comm., Vol. 77, 041801 (R) (2008) and “A new Two-photon Ghost Imaging Experiment with Distortion Study,” J. Mod. Opt., 54: 16, 2381-2392 (2007), both of which are hereby incorporated by reference, “ghost imaging” of remote objects by measuring reflected photons is reported.
“Ghost imaging” is a technique that allows a camera or image capture device to produce an image of an object which the camera or device does not directly receive; hence the terminology “ghost.” Early demonstrations of ghost imaging were based on the quantum nature of light; using quantum correlations between photon pairs to build an image of the unseen object. Generally speaking, “ghost imaging” comprises the characteristics of nonlocal multiphoton interference and imaging resolution that differs from that of classical imaging. Using correlated photons from photon pairs, a camera constructs an image using recorded pixels from photons that hit simultaneously at the object and the camera's image plane.
Two types of “ghost imaging” has been used experimentally since 1995; Type I uses entangled photon pairs as the light source and Type II uses a chaotic thermal light. Klyshko diagrams are shown for Type I and II sources are shown in FIGS. 2 and 3 respectfully.
Conventional line-of-sight imaging (graphically depicted in FIG. 1) lacks the ability to image target objects hidden by obstacles such as terrain, vegetation, buildings, and caves that place limitations on sensor positioning and field of view. Experiments have been performed proving that Ghost Imaging has abilities beyond those of classical imaging; including imaging through obscurants and turbulence.
FIG. 4 is a schematic diagram of an experimental optical device by Pittman, et al., as described in Pittman, et al. “Optical Imaging by Means of Two-photon Quantum Entanglement: Physical Review A, Vol. 52, No. 5, November 1995, hereby incorporated by reference, and hereinafter referred to as Pittman, et al. As described in Pittman, et al., signal and idler beams emerging from the SPDC crystal are sent in different directions so that coincidence detections may be performed between two distant photon counting detectors. An aperture placed in front of one of the detectors, for example, the letters UMBC, is illuminated by the signal beam through a convex lens. By placing the other detector at a distance prescribed by a “two-photon Gaussian thin lens equation” and scanning it in the transverse plane of the idler beam, a sharp magnified image of this aperture is observed in the coincidence counting rate, even though both detector's single counting rates remain constant.
The Pittman, et al. experimental setup is shown in FIG. 4. In the experiment a 2-mm-diameter beam from the 351.1-nm line of an argon ion laser is used to pump a nonlinear beta barium borate (BBO) (β-BaB204) crystal that is cut at a degenerate type-II phase-matching angle to produce pairs of orthogonally polarized signal (e-ray plane of the BBO) and idler (o-ray plane of the BBO) photons. The pairs emerge from the crystal nearly collinearly, with ωs=ωi=ωp/2. The pump is then separated from the slowly expanding down-conversion beam by a UV grade fused silica dispersion prism and the remaining signal and idler beams are sent in different directions by a polarization beam-splitting Thompson prism. The reflected signal beam passes through a convex lens with a 400-mm focal length and illuminates the (UMBC) aperture. Behind the aperture is the detector package D1, which consists of a 25-mm focal length collection lens in whose focal spot is a 0.8-mm-diam dry ice cooled avalanche photodiode. The transmitted idler beam is met by detector package D2, which consists of a 0.5-mm-diameter multimode fiber whose output is mated with another dry ice cooled avalanche photodiode. Both detectors are preceded by 83-nm-bandwidth spectral filters centered at the degenerate wavelength 702.2 nm. The input tip of the fiber is scanned in the transverse plane by two orthogonal encoder drivers, and the output pulses of each detector, which are operating in the Geiger mode, are sent to a coincidence counting circuit with a 1.8-ns acceptance window. By recording the coincident counts as a function of the fiber tip's transverse plane coordinate, an image of the UMBC aperture is seen as described further in Pittman, et al. The aperture containing the UMBC that was inserted in the signal beam (about 3.5×7 mm) is shown in the upper right, and the observed image (reportedly measured 7×14 mm) is shown beneath the aperture. Pittman, et al. demonstrated the viability of ghost imaging, it did not provide a viable solution for non-line-of-sight imaging, Current Ghost Imaging methods are based on having the object being imaged in the line-of-sight or field of view of the bucket detector.